Pressure pouring in a vacuum environment

ABSTRACT

An apparatus for melting and casting high-temperature, heatresistant alloys in a vacuum environment. The apparatus includes an induction-heated furnace having an upwardly extending pouring spout mounted in a vacuum chamber, a boron nitride pouring nozzle, a bottom fill permeable refractory mold, and means whereby the molten metal is forced under pressure into the mold and allowed to solidify. In the casting process, the metal is melted under a vacuum, an evacuated bottom fill mold is seated on the pouring nozzle, and the molten metal is forced under pressure up into the mold and allowed to solidify all under vacuum conditions to produce a precision casting of superior quality.

. United States Patent Bly et a1.

[451 Jan. 18, 1972 1,999,961 3,201,837 3,410,332 11/1968 Woodburn et a1.

PRESSURE POURING IN A VACUUM ENVIRONMENT Kenneth B. Bly, Bloomfield Hills; Donald S. Mills, Southfield, both of Mich.

Assign'ee: General Motors Corporation Filed: Aug. 4, 1969 Appl. No.: 847,118

Inventors:

US. Cl ..l64/258, 164/65, 164/68, I

16 4/119, 164/136, 164/335, 266/34 V ..B22d 27/16 ..l64/61, 63, 65, 66, 68,113, 164/119, 120, 133, 136, 253, 254, 255, 256, 257,

Int. Cl

Reierences Cited UNITED STATES PATENTS 4/1935 Daesen et a1. ..164/1 19 8/1965 Sylvester FOREIGN PATENTS OR APPLICATIONS '252,397 6/1964 Australia ..l64/306 8,155 11/1959 Japan ..164/257 Primary Examiner-J. Spencer Overholser Assistant Examiner-John E. Roethel Attorney-Stanley C. Thorpe and Peter P. Kozak [57] ABSTRACT An apparatus for melting and casting high-temperature, heatresistant alloys in a vacuum environment. The apparatus includes an induction-heated furnace having an upwardly extending pouring spout mounted in a vacuum chamber, a boron nitride pouring nozzle, a bottom fill permeable refractory mold, and means whereby the molten metal is forced under pressure into the mold and allowed to solidify. 1n the casting process, the metal is melted under a vacuum, an evacuated bottom fill mold is seated on the pouring nozzle, and the molten metal is forced under pressure up into the mold and allowed to solidify all under vacuum conditions to produce a precision casting of superior quality.

2 Claims, 3 Drawing Figures PATENTEU mm a ma SHEET 2 UF 2 m M M 1 lhdflhA t 7 MW 2 a, mu m f 0 @H (W 9 v m w a PRESSURE POURING IN A VACUUM ENVIRONMENT This invention relates generally to an apparatus for melting and casting high-temperature, heat-resistant alloys in a vacuum environment to make improved castings. The invention is particularly applicable to casting turbine blades and the .like for use in gas turbine engines and the like. The applicable alloy group which is invention is particularly suited are generally nickel-based alloys containing 40 to 80 percent by weight nickel, to 20 percent chromium, and the balance otherconstituents; and cobalt-based alloys containing 45 to 70 percent cobalt, to 30 percent chromium, and the balance other constituents. In the case of the nickel-based alloys, the other constituents may include up to 10 percent molybdenum, up to 5.5 percent titanium, up to 6.5 percent aluminum, up to 3 percent columbium, up to 9 percent tantalum, up to 13.5 percent tungsten, up to 2 percent hafnium, up to 1 percent rhenium, up to 1.5 percent vanadium, up to percent cobalt, up to 3 percent iron, and minor amounts of cesium, boron, zirconium, silicon, manganese, and impurities in the form of sulfur, copper, and phosphorus. In the case of cobalt-based'alloys, the other constituents may include up to 7 percent molybdenum, up to 10 percent tantalum, up to 16 percent tungsten, up to 16 percent nickel, up to 3 percent iron, minor amounts of carbon, manganese, silicon, titanium, boron, and zirconium, and impurities in the form of iron, sulfur, phosphorus, and copper. Castings of these alloys are used in applications requiring high strength and oxidation resistance up to a temperature of about 2,000 F. such as in gas turbine engines where the cast articles are subjected to extended periods of service at elevated temperatures under variable conditions. In addition to high-temperature strength and oxidation resistance, castings of these alloys for use in gas turbine engines and the like must be relatively free of oxide inclusion defects and have a relatively high degree of precision.

, It is, accordingly, the principal object of this invention to provide an apparatus for melting and casting high-temperature, heat-resistant alloys in a vacuum to produce precision, defect-free castings.

In general, these and other objects are accomplished by heating and melting a high-temperature, heat-resistant alloy undera vacuum environment in an induction-heated furnace which includes a crucible, an upwardly extending pouring spout externally of the crucible and communicating with the bottom of the crucible at one end, and a boron nitride pouring spout seated on the opposite end of the pouring spout, and then pressurizing the molten metal with an inert gas whereby the metal is forced at a controlled rate up the pouring spout against the force of gravity and into a bottom fill refractory mold seated on the nozzle, all while the vacuum environment is maintained. Melting and pouring under vacuum conditions offers the obvious advantage of removing oxygen and other gases from the molten metal as well as protection from contamination during melting and pouring. The vacuum renders the refractory mold gas-free and removes gases evolving from the refractory mold when the molten metal comes in contact with the mold. The pressure pouring and bottom filling techniques embodied by this invention offer the advantages of filling the mold at a controlled rate in a nonturbulent manner to produce a sounder casting with a substantial elimination of riser waste.

Other objects and advantages will become apparent when reference is made to the following description and accompanying drawings wherein:

FIG. 1 is a fragmentary cross-sectional view of an arrangement for casting of metals as embodied by this invention;

FIG. 2 is a view taken at line 2-2 of FIG. 1; and

FIG. 3 is an enlarged cross-sectional view of a portion of FIG. 1 and showing the mold seating arrangement.

Referring now to the drawings, particularly to FIG. 1, it will be observed that there is included in the melting and casting arrangement a tnain vacuum chamber 10 having a smaller vacuum lock compartment 12 mounted thereon and a slide valve 14 for selectively establishing communication between the vacuum lock compartment 12 and the vacuum chamber 10.

In the chamber 10 is an induction furnace 15 mounted on a platform 16 having wheels 17 movable on a set of tracks 18 so as to be movable within the chamber 10 by means of a suitable hydraulic arrangement 19. The induction furnace 15 comprises a melting crucible 20 adapted to hold molten metal 21 and an external pouring spout 22 which communicates with the bottom of the melting crucible 20 and extends upwardly to the top of furnace 15 terminating in an annular seat 23. Seated and clamped on the annular seat.23 is a refractory pouring nozzle 25 which will be more fully described later. Both the melting crucible 20 and the pouring spout 22 are surrounded by induction coils 24.for heating and melting the metal 21 in the melting crucible 20 and the pouring spout 22. As may best be seen in FIG. 2, the melting crucible 20 is provided with a sealed cover 26 removable by means of a slide arrangement 27 and having an opening 28 therethrough for receiving a tube 30 which protrudes through an opening 32 in the wall of the vacuum chamber 10 and isattached to a source of inert gas 34 such as argon by suitable conduit and valve means 36.

The main vacuum chamber 10 is provided with a sealed door 38 whereby access can be gained to the melting furnace and the vacuum lock compartment 12 is also provided with a sealed door 40 for the placing of molds 42 or charges of metal into the vacuum lock compartment 12 and for removing the molds 42 on completion of the casting operation. Associated with the vacuum lock compartment 12 is a piston-and-rod means 44 vertically reciprocable between the vacuum lock compartment 12 and the main vacuum chamber 10 having attached at its end a suitable holding means 46 such as a claw which can be operated to hold or release the mold 42.

The main vacuum chamber 10 also includes suitable means (not shown) for tipping the induction furnace 15 if desired to a position shown by the dotted lines, and a cast iron vessel 48 for receiving the remaining molten metal after casting.

The arrangement as illustrated in FIG. 1 also includes means for evacuating the main vacuum chamber 10 comprising a vane-type mechanical pump 50 serially connected by means of a conduit 51 to a booster pump (Roots type) 52 and to the foreline 53 of a vacuum diffusion pump 54. The diffusion pump 54 is connected to the vacuum chamber 10 through a valve assembly 55 in a conduit 56 for at times terminating communication therethrough. A second vacuum pumping system consisting of a mechanical vacuum pump 57 and a mechanical booster pump (Roots type) 58 are provided for evacuating the vacuum lock compartment. A valve 60 is provided inthe conduit 59 for at times terminating communication therethrough. The main vacuum chamber 10 is provided with a high-capacity vacuum diffusion pump because of its large volume and the degree of vacuum required to maintain a relatively low pressure.

OPERATION With doors 38 and 40 open a charge of cold metal ingots is placed in the melting crucible 20 in the furnace 15. The door 38 to the melting chamber 10 is closed and sealed and the valve 14 is closed thereby shutting off communication between the chamber 10 and the compartment 12. The chamber 10 is then evacuated to. the pressure in the range of 3 to 8X10 torr at which time melting of the ingots is begun by supplying power to the induction coils 24.

A precision refractory mold which in the preferred embodiment of this invention is of the permeable-shell-type utilizing the lost-wax" technique consisting mainly of zircon refractory particles with a colloidal silica binder, is preheated in a furnace at l,4502,l00 F. for 0.5 to 3.5 hours. After the metal charge has been completely melted the mold 42 is loaded in the jaws 46 of the rod 44 which is now in a raised position, the door 40 is closed and sealed and the vacuum lock compartment 12 is evacuated to a pressure equal that in the main vacuum chamber 10. If desired the mold 42 can be paced in a mold carrier 45 to aid in holding the mold in the jaws 46 of the rod 44.

When it is desired to pour a casting the induction furnace is moved to a pour position wherein the pouring spout 22 and nozzle 25 are directly beneath the mold 42 and the rod 44. The metal is raised to the correct pouring temperature which in the preferred embodiment is in the range of 2,700 F. to 1,900 F., the slide valve 14 is opened, and the mold is lowered and seated on the refractory pouring nozzle 25, preferably formed of boron 'nitride, which is clamped to the pouring spout 22.

Referring to FIG. 3 wherein the boron nitride pouring spout is shown in greater detail it will be observed that the pouring spout arrangement includes the annular seat 23 having an inwardly conical taper 61, a tubular boron nitride nozzle 25 having a conical taper 62 corresponding to that of the pouring spout 22 and adapted to seat on the annular end 23, and an asbestos seal 63 between the nozzle and the pouring spout. The asbestos seal prevents metal leakage during casting and allows a small amount of movement of the nozzle on the seat to compensate for any misalignment of the mold 42 with respect to the pouring spout 22. The mold 42 has a downwardly projecting conical portion 64 adapted to tightly seat on a corresponding conical portion 65 of the boron nitride nozzle. To prevent any metal leakage the mold is held in place by the weight of the mold rod assembly 44 which is approximately 70 pounds whereby the entrance to the mold is tightly sealed at its circumference. The assembly also includes a metal heat sink and clamp 66, for example, copper which surrounds the nozzle 25 and serves a dual purpose of holding the nozzle in place and of removing heat from the nozzle at a controlled rate so as to define a region above the heat sink where the metal may solidify in the nozzle and to control the rate of metal solidification. The inner surface of the nozzle is tapered to permit easy removal of the solidified metal 68 in the nozzle following the casting operation. Boron nitride is used in the preferred embodiment of this invention because of its good thermal stability and relatively high degree of resistance to attack by the molten metal at the casting temperatures previously recited. An important feature of the nozzle arrangement is that although the boron nitride shows a relatively long life, when necessary, it may be easily replaced.

After the mold is properly seated the pressure cover 26 is lowered over the melting crucible and argon gas is introduced over the molten metal 21 in the crucible at a pressure between 200 and 500 mm. of mercury to force the molten metal up the pouring spout through the boron nitride nozzle and into the mold. The molten metal rises and fills the mold under a pressure of only 200 to 500 mm. of mercury because of the differential pressure between the pressure in the mold cavity of 38 l0' torr and the pressure on the molten metal of 200-500 mm. of mercury. The seal between the nozzle and the annular seat and between the nozzle and the mold will resist the pressure of the molten metal and prevent leakage. The gas pressure is carefully controlled to control the rate of mold fill so that the molten metal enters the mold in a nonturbulent manner. For example, when an 8-pound casting is being cast the rate of mold fill is in the range of 1.5 to 6 seconds. Since the mold is permeable and is filled in a nonturbulent manner outgases released from the mold on the introduction of the molten metal into the mold are driven ahead of the molten metal and removed through the mold by the vacuum system which permits the molten metal to enter the mold and to fill the intricate detail of the mold without entrapping the outgases whereby a precision casting 67 free of oxide inclusions is produced. In addition, since the argon gas is introduced only over the surface of the melting crucible rather than in the entire vacuum chamber, the consumption of argon gas per casting is only about 2 cubic feet. Since the mold is very close to the source of molten metal and its induction-heating field, the rate of solidification of the metal in the nozzle and the molten metal in the mold above the nozzle can be very accurately controlled by adjusting the power supplied to the induction coil. The argon gas pressure is maintained while the metal is solidifying in the mold and after the period of solidification the pressure is released from the molten metal and the remaining molten metal recedes in the pouring spout leaving a small sprue 68 of solidified metal attached to the casting.

After solidification the mold is raised into the vacuum lock compartment 12 by means of the rod 42 and the valve 14 is closed. Pressure in the vacuum lock compartment 12 is brought to that of the atmosphere following which door 40 is opened and the mold 42 is removed. Another mold is.then placed in the jaws 46 and the process repeated.

Periodically throughout the process it will become necessary to recharge the melting crucible 20. Although it is usually not necessary to do so, if desired, the furnace can be tipped to the position shown by the dotted lines in FIG. 1 and the metal remaining in the crucible 20 can be poured into the cast iron vessel 48 to leave the crucible clean for another charge. The furnace 15 is then moved on the tracks 18 until the melting chamber 20 is directly beneath the rod 44. A charge of metal is then placed in the jaws 46 of the rod 44 and the door 40 is closed, the vacuum lock compartment 12 is evacuated, the slide valve 14 opened, and the rod 44 is lowered to insert the charge into the melting chamber 20. The rod 44 is then raised back into the vacuum lock compartment 12, slide valve 14 is again closed, and the furnace 16 is moved back to the pouring position whereupon the casting process is resumed.

It will be apparent that the process described above has a number of important advantages. As described above the molten metal is moved into the mold slowly and nonturbulently in the form of a substantially plane front so that the outgases are driven ahead of the metal and removed by the vacuum system rather than being occluded in the metal as in conventional gravity pouring techniques. Controlled filling of the mold also ensures that the details of the mold will be reproduced in every casting thereby markedly reducing the rejection rate. Further, since a bottom fill technique is employed only a small riser is fonned thereby eliminating a large amount of riser waste. Another important aspect of the process is that the castings are subject to continuous pressure until complete solidification has been substantially completed whereby shrinkage and porosity of the castings is minimized. The use of the boron nitride nozzle offers significant advantages in the pressure pouring technique in that boron nitride is substantially unaffected by the molten metal, the molten metal does not adhere to the surface of the nozzle, and the rate of heat flow from the nozzle can be accurately controlled so as to obtain a controlled rate of solidification.

Although the invention has been described in terms of a specific embodiment, it will be understood that various modifications may be made within the scope of the invention.

We claim:

1. A casting apparatus comprising, in combination,

a sealed melting and mold pouring chamber,

a sealed vacuum lock compartment mounted on said chamber,

valve and conduit means for establishing communication between said melting chamber and said vacuum lock compartment,

means for evacuating said melting chamber during melting and pressure pouring and for selectively evacuating said vacuum lock compartment,

a furnace mounted and movable within said chamber between a charging position and a pressure pouring position containing a melting crucible adapted to hold molten metal and having a removable sealed cover,

an upwardly extending pouring spout externally of said crucible communicating with the bottom of said crucible and having at its other end an annular seat portion having a conical taper,

induction heating coils surrounding said melting crucible and said pouring spout,

removable tubular refractory nozzle having at one end a conical taper adapted to mate with said conical taper of said seat and to seat of said seat portion and having a conical taper at the opposite end thereof,

heat-resistant resilient sealing means between said seat and said nozzle,

a permeable refractory mold having a downwardly extending conical portion adapted to mate with said conical taper at said opposite end of said nonle and to seat on said nozzle,

a metallic heat removal and clamping means adapted to hold said nozzle on said seat and to withdraw heat from said nozzle at a desired rate thereby defining a region where said molten metal may solidify in said nozzle,

said nozzle being movable on said sealing means to compensate for any misalignment between said mold and said seat to provide a passage between said pouring spout and said mold,

said sealed cover having an opening therethrough and receiving a tube means communicating with a source of pressurized gas outside of said chamber for increasing the pressure on the surface of said molten metal in said crucible when said cover is in a closed position to force said molten metal against the force of gravity up said spout through said nozzle and into said mold at a controlled rate whereby said metal enters said mold with a substantially plane front and to hold said metal in said mold during solidification of said metal, and

holding means associated with said vacuum lock compartment vertically aligned with said valve and conduit means and being reciprocal between said compartment and said chamber through said valve and conduit, for seating said mold on said nozzle, for holding said mold on said nozzle during pressure pouring, and for raising said mold from said nozzle and into said compartment after said metal has solidified in said mold.

2. The apparatus of claim 1 wherein said nozzle is formed of boron nitride.

I I II l I 0- UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO... 3,635,791 Dated January 18, 1972 Kenneth B. Bly and Donald S. Mills I Inventork's) It certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below: 7

Column 1, line 7, after "group" insert to delete "is", first occurrence, and substitute therefor this I column 2, line 73, "paced" should read placed column 3,

line 6, "l,900F. should read 2,900F.

' Claim 1, line 74, "of" should read on Signed and sealed this 27th day of 'June' 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Attesting Officer ROBERT GOTTSCHALK Commissioner of Patents 

1. A casting apparatus comprising, in combination, a sealed melting and mold pouring chamber, a sealed vacuum lock compartment mounted on said chamber, valve and conduit means for establishing communication between said melting chamber and said vacuum lock compartment, means for evacuating said melting chamber during melting and pressure pouring and for selectively evacuating said vacuum lock compartment, a furnace mounted and movable within said chamber between a charging position and a pressure pouring position containing a melting crucible adapted to hold molten metal and having a removable sealed cover, an upwardly extending pouring spout externally of said crucible communicating with the bottom of said crucible and having at its other end an annular seat portion having a conical taper, induction heating coils surrounding said melting crucible and said pouring spout, removable tubular refractory nozzle having at one end a conical taper adapted to mate with said conical taper of said seat and to seat of said seat portion and having a conical taper at the opposite end thereof, heat-resistant resilient sealing means between said seat and said nozzle, a permeable refractory mold having a downwardly extending conical portion adapted to mate with said conical taper at said opposite end of said nozzle and to seat on said nozzle, a metallic heat removal and clamping means adapted to hold said nozzle on said seat and to withdraw heat from said nozzle at a desired rate thereby defining a region where said molten metal may solidify in said nozzle, said nozzle being movable on said sealing means to compensate for any misalignment between said mold and said seat to provide a passage between said pouring spout and said mold, said sealed cover having an opening therethrough and receiving a tube means communicating with a source of pressurized gas outside of said chamber for increasing the pressure on the surface of said molten metal in said crucible when said cover is in a closed position to force said molten metal against the force of gravity up said spout through said nozzle and into said mold at a controlled rate whereby said metal enters said mold with a substantially plane front and to hold said metal in said mold during solidification of said metal, and holding means associated with said vacuum lock compartment vertically aligned with said valve and conduit means and being reciprocal between said compartment and said chamber through said valve and conduit, for seating said mold on said nozzle, for holding said mold on said nozzle during pressure pouring, and for raising said molD from said nozzle and into said compartment after said metal has solidified in said mold.
 2. The apparatus of claim 1 wherein said nozzle is formed of boron nitride. 